In a conventional alphanumeric display, individual segments arranged in some predefined pattern are selectively energized to visually represent a desired character. The most common type of display includes at least seven discrete segments arranged so as to visually represent a numeral eight if all of the segments are simultaneously energized. Although this type of display can represent a limited number of alpha characters, it is most often employed to represent the numbers zero through nine. Substantially the same seven segment configuration (although fabricated very differently for the two types of display technologies) can be used for both light emitting diode (LED) displays and liquid crystal displays (LCDs).
There are several advantages that justify using LCDs rather than LEDs in instrument display panels. Particularly important in portable, battery powered instruments is the intrinsically lower power requirement of the liquid crystal technology. In addition, relatively complex arrangements of graphic icons and alphanumeric character displays can readily be configured on a common substrate to produce a complete LCD panel for an instrument. By contrast, LED displays are more directed to representing characters than graphic icons and are not as easily fabricated in complex, integrated panels.
Each segment of an LCD corresponds to similarly shaped electrically conductive regions applied to the front and rear surfaces of the display. These electrically conductive regions are coupled to a control circuit that supplies a voltage appropriate to modify the optical characteristics of a liquid crystal layer disposed between the front and rear surfaces. The electrical signal applied to these regions causes the liquid crystal layer to become more opaque, so that a character or graphic icon corresponding to the shape of the electrically conducting regions is visible. Optionally, background lighting can be provided for an LCD to produce greater contrast so that the characters or icons are more easily visible at low ambient light levels.
There are certain applications in which a failure of one of the segments that defines a character or graphic icon may have life-threatening consequences. For example, a display panel on a medical instrument may indicate certain critical operating parameters to an operator of the instrument. Clearly, in this instance, it is very important to avoid errors in reading the displayed data. An error caused by the failure of a segment in a numeric display character would be particularly serious if the character is the most significant digit of a critical displayed value. For instance, failure of the center segment in a seven segment character of the display would cause an "8" to visually appear as an "0". A medical practitioner relying on the incorrect displayed reading caused by such a failure might use the instrument in a manner that hams a patient. Accordingly, for any critical displayed parameter on a medical instrument or on instruments used in other critical applications, designers have recognized the importance of detecting a display failure so that the user is alerted and does not rely upon an incorrect value.
One way to insure that a failure in a critical display character is detected is to duplicate the entire displayed parameter, so that two nominally identical values for the parameter are indicated in separate displays. If the two displayed values are different, the user is supposed to recognize that a failure has occurred in one of the duplicated displays. However, because the two displayed values are spatially separate, such difference may go unnoticed. Moreover, space limitations on a display panel often render it impractical to provide duplicate values of a parameter, and this solution to the problem is inelegant at best.
If LEDs are used for such critical displayed parameters, the electrical current to each digit of the display can be monitored and compared to an expected value in a look-up table that relates each of the possible characters to the electrical current draw required for that character, based on the number of segments that must be selectively energized to represent it. For example, a numeral "1" requires that only two segments be energized (in a seven segment display), and the look-up table defines a current corresponding to that required by the two segments. If less current is detected, at least one of the two segments may have failed and a monitoring circuit alerts the operator with a visual and/or audible alarm. Similarly, if current to a segment is detected when that segment should not be energized, the monitoring circuit also detects a failure.
Unfortunately, the low current requirements of LCDs make it practically impossible to detect a failed segment in an LCD character based on electrical current measurements. Consequently, although LCDs are almost uniformly preferred for display panels because of the variety of graphic options that can be included and because of their low power requirement, LEDs are often used for the display of critical parameter values, simply to ensure that the failure of a segment in the critical display can be detected by the current monitoring method. Therefore, both an LCD display panel and LEDs must be used on such instruments. It should be evident that it would be preferable to use only LCDs in a display panel, if failure of a segment of any LCD characters was clearly evident.
In a copending, commonly assigned patent application, Ser. No. 08/125,508, filed on Sep. 21, 1993, entitled "Display Having Redundant Segments" (R. Poli and A. Ravid), a solution to the above-noted problem is disclosed. In the invention disclosed in this reference, at least some of the segments comprising each display character are configured as pairs of segments, wherein one of the pair of segments is redundant, extending either parallel or end-to-end relative to the other segment of the pair. If one of the pair of segments should fail to be visually perceptible when activated or by remaining visually perceptible when de-activated, the failure will be immediately visually evident to the user because the other segment of the pair will continue to operate properly. Any difference between the operation of the two segments comprising each pair is visually evident and thus serves to warn a user that a problem or failure in the display has occurred. If fewer than all of the segments are formed as pairs of segments, the segments selected for redundant configuration are those in which failure of the segment, if not evident, could cause the resulting character to visually represent an incorrect character that might not otherwise be evident as an incorrect character.
Although the display circuit comprising the above-noted invention represents a solution to the problem of detecting failed segments in a display and yet representing a desired character even when a failure of one segment has occurred, there are some applications in which it may be too expensive to fabricate pairs of redundant segments, or in which the display panel does not have sufficient area or edge connect terminations to support the greater number of conducting traces and terminals required for such a display circuit. Accordingly, another approach that is simpler to fabricate and requires fewer conductors would be preferable.